arXiv subjects

Roger Proksch

Publications and source records attributed to Roger Proksch.

At least 19 recordsLinked to original sources

arXiv2026

Correlated domain and crystallographic orientation mapping in uniaxial ferroelectric polycrystals by interferometric vector piezoresponse force microscopy

Ongoing advances in scanning probe microscopy techniques are continually expanding the possibilities for nanoscale characterization and correlated studies of functional materials. Here, we demonstrate how a recent extension of piezoresponse force microscopy (PFM), known as interferometric vector PFM, can be utilized for simultaneously mapping the local crystallographic orientations and the domain structure of distributed grains in uniaxial ferroelectric polycrystals. By shifting the laser beam position on the cantilever, direction-dependent piezoresponse signals are acquired analogous to classical vector PFM, but without the need to rotate the sample. Using polycrystalline ErMnO$_{3}$ as a model system, we demonstrate that the reconstructed piezoresponse vectors correlate one-to-one with the crystallographic orientations of the micrometer-sized grains, carrying grain-orientation and domain-related information. We establish a versatile approach for rapid, multimodal characterization of polycrystalline uniaxial ferroelectrics, enabling automated, high-throughput reconstruction of polarization and grain orientations with nanoscale precision.

cond-mat.mtrl-sci↗

arXiv2026

Hidden Information in Force Curves: Transient- and Brownian-Driven Dynamics

Force distance curves (FCs) are direct probes of tip sample interactions in atomic force microscopy, but their dynamic content is often suppressed by filtering, averaging and quasistatic analysis. Indeed, in the forty years since the invention of the AFM, this hidden dynamic signal has been largely ignored, with exceptions restricted to the large oscillations associated with relatively large adhesive separations (snap from contact) or to time averaged approaches that carry their own challenges. Using high bandwidth interferometric detection, we directly measure fast FCs, including short lived cantilever oscillations during snap in, contact and pull off. Two excitation mechanisms coexist: thermal fluctuations and transients generated by rapid changes in the tip sample boundary condition. Cycle resolved frequency and amplitude estimators provide time local observables of the evolving cantilever and sample system within single FCs, creating a pixel by pixel mechanical contrast channel that requires no external broadband excitation, resonant tracking or multi pass imaging. A state dependent harmonic oscillator model incorporating van der Waals attraction, hysteretic capillary bridge formation and rupture, hydration mediated damping and repulsive contact stiffness partitions each FC into five interaction regimes and reproduces the observed responses. In repulsive contact, frequency shifts yield stiffness maps consistent with contact-mechanics estimates, enabling high throughput stiffness mapping and localization of transient tip sample events from individual pixels. More broadly, this hitherto hidden signal may pave the way for new ways of studying and exploiting Brownian motion in a variety of systems, providing the capability to follow time resolved (non) Markovian and (non) ergodic dynamics mediated by the tip sample interaction, including single event dynamics.

cond-mat.mes-hall↗

arXiv2025

Lateral force microscopy calibration using an interferometric atomic force microscope

A new method is introduced for calibrating lateral force as measured by an atomic force microscope (AFM), making use of both an interferometric detector and an optical beam detector on the same instrument. The method may be implemented automatically and performed with minimal user input. The microscope has the capability to measure the probe tip height in situ, which allows for a complete lateral force calibration without changing the sample or probe. Two options for lateral force measurements are described wherein the two detectors are alternately used to measure normal and lateral forces, and methods for applying the calibration protocol for both alternatives are provided. The tip height measurement is validated by direct comparison with an electron micrograph and they are generally in agreement to within 1.4 microns. For most cantilevers tested, the complete lateral calibration method is consistent with the wedge calibration method to within the intrinsic uncertainty of the wedge method.

cond-mat.mes-hall↗

arXiv2024

Reward based optimization of resonance-enhanced piezoresponse spectroscopy

Dynamic spectroscopies in Scanning Probe Microscopy (SPM) are critical for probing material properties, such as force interactions, mechanical properties, polarization switching, and electrochemical reactions and ionic dynamics. However, the practical implementation of these measurements is constrained by the need to balance imaging time and data quality. Signal to noise requirements favor long acquisition times and high frequencies to improve signal fidelity. However, these are limited on the low end by contact resonant frequency and photodiode sensitivity, and on the high end by the time needed to acquire high-resolution spectra, or the propensity for samples degradation under high field excitation over long times. The interdependence of key parameters such as instrument settings, acquisition times, and sampling rates makes manual tuning labor-intensive and highly dependent on user expertise, often yielding operator-dependent results. These limitations are prominent in techniques like Dual Amplitude Resonance Tracking (DART) in Piezoresponse Force Microscopy (PFM) that utilize multiple concurrent feedback loops for topography and resonance frequency tracking. Here, a reward-driven workflow is proposed that automates the tuning process, adapting experimental conditions in real time to optimize data quality. This approach significantly reduces the complexity and time required for manual adjustments and can be extended to other SPM spectroscopic methods, enhancing overall efficiency and reproducibility.

cond-mat.mtrl-sci↗

arXiv2024

3D Nanoscale Electromechanical Imaging with Interferometric Atomic Force Microscopy

Forces acting between an Atomic Force Microscope (AFM) tip and sample are three dimensional. Despite this, most AFM force measurements are confined to one or two dimensions. Extending AFM force measurements into three dimensions has previously required complex, difficult and time-consuming workflows. Here, we demonstrate an accurate, interferometric method for quantifying the full, three-dimensional response of an AFM tip to localized forces. We demonstrate this approach on a series of piezoelectric materials and show that this approach yields quantitative 3D measurement independent of the sample orientation beneath the tip. This approach simplifies existing, angle-resolved piezoresponse force microscopy (PFM) techniques. Our measurements benefit from the greatly reduced noise floor (5 fm per root Hz) and intrinsic accuracy of our interferometric measurements. One important result is that the vertical piezo sensitivity was systematically 2 to 3 times larger than the in-plane piezo sensitivities. A simple analysis of vertical and lateral contact stiffnesses, due to the difference in the Young (vertical) and Shear (lateral) sample yields a factor of 2.5, in good agreement with the measurements. While this work was confined to ferroelectric materials, it provides a general workflow and framework for other AFM based mechanical measurements.

cond-mat.mes-hall↗

arXiv2024

Machine Learning-Based Reward-Driven Tuning of Scanning Probe Microscopy: Towards Fully Automated Microscopy

Since the dawn of scanning probe microscopy (SPM), tapping or intermittent contact mode has been one of the most widely used imaging modes. Manual optimization of tapping mode not only takes a lot of instrument and operator time, but also often leads to frequent probe and sample damage, poor image quality and reproducibility issues for new types of samples or inexperienced users. Despite wide use, optimization of tapping mode imaging is an extremely hard problem, ill-suited to either classical control methods or machine learning. Here we introduce a reward-driven workflow to automate the optimization of SPM in the tapping mode. The reward function is defined based on multiple channels with physical and empirical knowledge of good scans encoded, representing a sample-agnostic measure of image quality and imitating the decision-making logic employed by human operators. This automated workflow gives optimal scanning parameters for different probes and samples and gives high-quality SPM images consistently in the attractive mode. This study broadens the application and accessibility of SPM and opens the door for fully automated SPM.

cond-mat.mes-hall↗

arXiv2024

Integration of Scanning Probe Microscope with High-Performance Computing: fixed-policy and reward-driven workflows implementation

The rapid development of computation power and machine learning algorithms has paved the way for automating scientific discovery with a scanning probe microscope (SPM). The key elements towards operationalization of automated SPM are the interface to enable SPM control from Python codes, availability of high computing power, and development of workflows for scientific discovery. Here we build a Python interface library that enables controlling an SPM from either a local computer or a remote high-performance computer (HPC), which satisfies the high computation power need of machine learning algorithms in autonomous workflows. We further introduce a general platform to abstract the operations of SPM in scientific discovery into fixed-policy or reward-driven workflows. Our work provides a full infrastructure to build automated SPM workflows for both routine operations and autonomous scientific discovery with machine learning.

cond-mat.mtrl-sci↗

arXiv2023

Nanoscale rheology: Dynamic Mechanical Analysis over a broad and continuous frequency range using Photothermal Actuation Atomic Force Microscopy

Polymeric materials are widely used in industries ranging from automotive to biomedical. Their mechanical properties play a crucial role in their application and function and arise from the nanoscale structures and interactions of their constitutive polymer molecules. Polymeric materials behave viscoelastically, i.e. their mechanical responses depend on the time scale of the measurements; quantifying these time-dependent rheological properties at the nanoscale is relevant to develop, for example, accurate models and simulations of those materials, which are needed for advanced industrial applications. In this paper, an atomic force microscopy (AFM) method based on the photothermal actuation of an AFM cantilever is developed to quantify the nanoscale loss tangent, storage modulus, and loss modulus of polymeric materials. The method is then validated on a styrene-butadiene rubber (SBR), demonstrating the method's ability to quantify nanoscale viscoelasticity over a continuous frequency range up to five orders of magnitude (0.2 Hz to 20,200 Hz). Furthermore, this method is combined with AFM viscoelastic mapping obtained with amplitude-modulation frequency-modulation (AM-FM) AFM, enabling the extension of viscoelastic quantification over an even broader frequency range, and demonstrating that the novel technique synergizes with preexisting AFM techniques for quantitative measurement of viscoelastic properties. The method presented here introduces a way to characterize the viscoelasticity of polymeric materials, and soft matter in general at the nanoscale, for any application.

cond-mat.mes-hall↗

arXiv2023

Accurate Vertical Nanoelectromechanical Measurements

Accurate measurements of the nanoscale electromechanical coupling in materials, including piezo and ferroelectrics, twisted 2D layers, and biological systems is of both fundamental scientific and applied importance. Piezoresponse Force Microscopy (PFM) is capable of detecting strains in these materials, down to the picometer range. Following the emergence of weaker materials, the smaller signals associated with them have revealed various crosstalk challenges that have limited the accuracy of measurements. Previous work demonstrated that the use of an interferometric displacement sensor (IDS) positioned appropriately above the tip of the cantilever (x/L~1), where x is the spot position and L is the cantilever length, has enabled sensitive and artifact-free electromechanical measurements. A similar approach has been employed in removing unwanted electrostatic and in-plane response contributions for the optical beam deflection (OBD) measurement technique commonly used in most atomic force microscopes. In the present study, extensive automated sub-resonance spot position dependent PFM measurements were conducted on periodically poled lithium niobate (PPLN). In this work, both IDS and OBD responses were measured simultaneously, allowing direct comparisons of the two approaches. The IDS showed a blind spot at x/L~1, as expected. However, for OBD measurements, the blind spot's location exhibited wider variation, ranging from 0.15<x/L<0.61. Furthermore, the magnitudes of the amplitudes measured with IDS and OBD were typically different, sometimes approaching disagreement by a factor of two. These measurements have important implications, not only for the PPLN measured here, but for more complex and unknown samples that have a heterogeneous polarization and electrical characteristic.

cond-mat.mes-hall↗

arXiv2021

Disentangling ferroelectric wall dynamics and identification of pinning mechanisms via deep learning

Field-induced domain wall dynamics in ferroelectric materials underpins multiple applications ranging from actuators to information technology devices and necessitates a quantitative description of the associated mechanisms including giant electromechanical couplings, controlled non-linearities, or low coercive voltages. While the advances in dynamic Piezoresponse Force Microscopy measurements over the last two decades have rendered visualization of polarization dynamics relatively straightforward, the associated insights into the local mechanisms have been elusive. Here we explore the domain dynamics in model polycrystalline materials using a workflow combining deep learning-based segmentation of the domain structures with non-linear dimensionality reduction using multilayer rotationally-invariant autoencoders (rVAE). The former allows unambiguous identification and classification of the ferroelectric and ferroelastic domain walls. The rVAE discover the latent representations of the domain wall geometries and their dynamics, thus providing insight into the intrinsic mechanisms of polarization switching, that can further be compared to simple physical models. The rVAE disentangles the factors affecting the pinning efficiency of ferroelectric walls, offering insights into the correlation of ferroelastic wall distribution and ferroelectric wall pinning.

cond-mat.dis-nn↗

arXiv2020

Unsupervised Machine Learning Discovery of Chemical and Physical Transformation Pathways from Imaging Data

We show that unsupervised machine learning can be used to learn physical and chemical transformation pathways from the observational microscopic data, as demonstrated for atomically resolved images in Scanning Transmission Electron Microscopy (STEM) and ferroelectric domain structures in Piezoresponse Force Microscopy (PFM). To enable this analysis in STEM, we assumed the existence of atoms, a discreteness of atomic classes, and the presence of an explicit relationship between the observed STEM contrast and the presence of atomic units. In PFM, we assumed the uniquely-defined relationship between the measured signal and polarization distribution. With only these postulates, we developed a machine learning method leveraging a rotationally-invariant variational autoencoder (rVAE) that can identify the existing structural units observed within a material. The approach encodes the information contained in image sequences using a small number of latent variables, allowing the exploration of chemical and physical transformation pathways via the latent space of the system. The results suggest that the high-veracity imaging data can be used to derive fundamental physical and chemical mechanisms involved, by providing encodings of the observed structures that act as bottom-up equivalents of structural order parameters. The approach also demonstrates the potential of variational (i.e., Bayesian) methods for physical sciences and will stimulate the development of new ways to encode physical constraints in the encoder-decoder architectures, and generative physical laws, topological invariances, and causal relationships in the latent space of VAEs.

cond-mat.mtrl-sci↗

arXiv2019

Quantitative Electromechanical Atomic Force Microscopy

The ability to probe a materials electromechanical functionality on the nanoscale is critical to applications from energy storage and computing to biology and medicine. Voltage modulated atomic force microscopy (VM-AFM) has become a mainstay characterization tool for investigating these materials due to its unprecedented ability to locally probe electromechanically responsive materials with spatial resolution from microns to nanometers. However, with the wide popularity of VM-AFM techniques such as piezoresponse force microscopy (PFM) and electrochemical strain microscopy (ESM) there has been a rise in reports of nanoscale electromechanical functionality, including hysteresis, in materials that should be incapable of exhibiting piezo- or ferroelectricity. Explanations for the origins of unexpected nanoscale phenomena have included new material properties, surface-mediated polarization changes and/or spatially resolved behavior that is not present in bulk measurements. At the same time, it is well known that VM-AFM measurements are susceptible to numerous forms of crosstalk and, despite efforts within the AFM community, a global approach for eliminating this has remained elusive. In this work, we develop a method for easily demonstrating the presence of hysteretic (ie, false ferroelectric) long-range interactions between the sample and cantilever body. This method should be easy to implement in any VM-AFM measurement. We then go on to demonstrate fully quantitative and repeatable nanoelectromechanical characterization using an interferometer. These quantitative measurements are critical for a wide range of devices including mems actuators and sensors, memristor, energy storage and memory.

cond-mat.mes-hall↗

arXiv2017

Fast, High Resolution and Wide Modulus Range Nanomechanical Mapping with Bimodal Tapping Mode

Tapping mode atomic force microscopy (AFM), also known as amplitude modulated (AM) or AC mode, is a proven, reliable and gentle imaging mode with widespread applications. Over the several decades that tapping mode has been in use, quantification of tip-sample mechanical properties such as stiffness has remained elusive. Bimodal tapping mode keeps the advantages of single-frequency tapping mode while extending the technique by driving and measuring an additional resonant mode of the cantilever. The simultaneously measured observables of this additional resonance provide the additional information necessary to extract quantitative nanomechanical information about the tip-sample mechanics. Specifically, driving the higher cantilever resonance in a frequency modulated (FM) mode allows direct measurement of the tip-sample interaction stiffness and, with appropriate modeling, the setpoint-independent local elastic modulus. Here we discuss the advantages of bimodal tapping, coined AM-FM imaging, for modulus mapping. Results are presented for samples over a wide modulus range, from a compliant gel (~100 MPa) to stiff materials (~100 GPa), with the same type of cantilever. We also show high-resolution (sub-nanometer) stiffness mapping of individual molecules in semi-crystalline polymers and of DNA in fluid. Combined with the ability to remain quantitative even at line scan rates of nearly 40 Hz, the results demonstrate the versatility of AM-FM imaging for nanomechanical characterization in a wide range of applications.

cond-mat.mes-hall↗

arXiv2016

Calibration of higher eigenmodes of cantilevers

A method is presented for calibrating the higher eigenmodes (resonance modes) of atomic force microscopy cantilevers that can be performed prior to any tip-sample interaction. The method leverages recent efforts in accurately calibrating the first eigenmode by providing the higher-mode stiffness as a ratio to the first mode stiffness. A one-time calibration routine must be performed for every cantilever type to determine the power-law relationship between stiffness and frequency, which is then stored for future use on similar cantilevers. Then, future calibrations only require a measurement of the ratio of resonance frequencies and the stiffness of the first mode. This method is verified through stiffness measurements using three independent approaches: interferometric measurement, AC approach-curve calibration, and finite element analysis simulation. Power-law values for calibrating higher-mode stiffnesses are reported for three popular multifrequency cantilevers. Once the higher-mode stiffnesses are known, the amplitude of each mode can also be calibrated from the thermal spectrum by application of the equipartition theorem.

cond-mat.mes-hall↗

arXiv2015

Quantitative Measurements of Electromechanical Response with a Metrological Atomic Force Microscope

An ongoing challenge in atomic force microscope (AFM) experiments is the quantitative measurement of cantilever motion. The vast majority of AFMs use the optical beam deflection (OBD) method to infer the deflection of the cantilever. The OBD method is easy to implement, has impressive noise performance and tends to be mechanically robust. However, it represents an indirect measurement of the cantilever displacement, since it is fundamentally an angular rather than a displacement measurement. Here, we demonstrate a metrological AFM that combines an OBD sensor with a laser Doppler vibrometer (LDV) to enable accurate measurements of the cantilever velocity and displacement. The OBD/LDV AFM allows a host of quantitative measurements to be performed, including in-situ measurements of cantilever oscillation modes in piezoresponse force microscopy (PFM). As an example application, we demonstrate how this instrument can be used for accurate quantification of piezoelectric sensitivity, a longstanding goal in the electromechanical community.

cond-mat.mes-hall↗

arXiv2014

Photothermally Excited Contact Resonance Imaging in Air and Water

Contact Resonance Force Microscopy (CR-FM) is a leading AFM technique for measuring viscoelastic nano-mechanical properties. Conventional piezo-excited CR-FM measurements have been limited to imaging in air, since the "forest of peaks" frequency response associated with acoustic excitation methods effectively masks the true cantilever resonance. Using photothermal actuation results in clean contact resonance spectra, that closely match the ideal frequency response of the cantilever, allowing unambiguous and simple resonance frequency and quality factor measurements in air and liquids alike. This extends the capabilities of CR-FM to biologically relevant and other soft samples in liquid environments. We demonstrate CR-FM in air and water on both stiff silicon/titanium samples and softer polystyrene-polyethylene-polypropylene polymer samples with the quantitative moduli having very good agreement between expected and measured in both environments.

cond-mat.mes-hall↗

arXiv2014

In-situ Piezoresponse Force Microscopy Cantilever Mode Shape Profiling

The frequency-dependent amplitude and phase in piezoresponse force microscopy (PFM) measurements are shown to be a consequence of the Euler-Bernoulli (EB) dynamics of atomic force microscope (AFM) cantilever beams used to make the measurements. Changes in the cantilever mode shape as a function of changes in the boundary conditions determine the sensitivity of cantilevers to forces between the tip and the sample. Conventional PFM and AFM measurements are made with the motion of the cantilever measured at one optical beam detector (OBD) spot location. A single OBD spot location provides a limited picture of the total cantilever motion and in fact, experimentally observed cantilever amplitude and phase are shown to be strongly dependent on the OBD spot position for many measurements. In this work, the commonly observed frequency dependence of PFM response is explained through experimental measurements and analytic theoretical EB modeling of the PFM response as a function of both frequency and OBD spot location on a periodically poled lithium niobate (PPLN) sample. One notable conclusion is that a common choice of OBD spot location, at or near the tip of the cantilever is particularly vulnerable to frequency dependent amplitude and phase variations stemming from dynamics of the cantilever sensor rather than from the piezoresponse of the sample.

cond-mat.mes-hall↗

arXiv2013

Electrochemical Strain Microscopy of Silica Glasses

Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) are two related techniques that have had considerable success in nano-scale probing of functional material properties. Both measure the strain of the sample in response to a localized electric field beneath a sharp conductive tip. In this work, a collection of commercially available glass samples were measured with a variety of Si cantilevers coated with different conductive metals. In some cases, these glasses showed significant hysteresis loops, similar in appearance to those measured on ferroelectric materials with spontaneous permanent electric dipoles. The magnitude of the electrochemical strain and hysteresis correlated well with the molar percentage of sodium in the glass material, with high sodium (soda-lime) glass showing large hysteresis and fused silica (pure SiO2) showing essentially no hysteresis. The elephant-ear shape of the hysteresis loops correlated well with it originating from relaxation behavior, an interpretation verified by observing the temperature dependent relaxation of the ESM response. Cation mobility in a disordered glass should have a low diffusion constant. To evaluate this diffusion constant, the temperature of the glass was varied between room temperature and 200C. Vanishing hysteresis as the temperature increased was associated with a decrease in the relaxation time of the electrochemical response. The hysteretic behavior changed drastically in this temperature range, consistent with bound surface water playing a large role in the relaxation. This demonstrates the ability of ESM to differentiate cationic concentrations in a range of silica glasses. In addition, since glass is a common sample substrate for, this provides some clear guidance for avoiding unwanted substrate crosstalk effects in piezoresponse and electrochemical strain response measurements.

cond-mat.mes-hall↗